1. Field of the Invention
The present invention relates to video data display, and more particularly, to a method for controlling a display device to maintain a same aspect ratio of input video images and output images.
2. Description of the Prior Art
Display devices (such as TVs, LCD monitors, PDP monitors, and projectors, etc.) can be utilized to display static images or dynamic video. Display devices are one of the most popular electronic devices available on the market.
Different video formats usually have different resolutions and aspect ratios. Taking Video Graphics Array (VGA) format as an example, the resolution and aspect ratio of VGA format are 640*480 and 4:3 respectively. The resolution and aspect ratio of Super Extended Graphics Array (SXGA) format are 1280*1024 and 5:4 respectively. When the resolution of a display device is different from that of received video signals, it is necessary to scale the received input video image first in order to display the video image correctly.
In general, video scaling will maintain a full frame display as far as possible. For example, if the resolution of an input video image is 640*480, and the resolution of a display device is 1280*1024, then the horizontal part of the input frame will be scaled up to be 2 times larger and the vertical part of the input frame will be scaled up to be 2.133 times larger for the full frame display. This scaling method for maintaining the full frame display does not require a large buffering space and it can maintain balance between the input data stream and the output display data stream via a proper controlling scheme. However, there are problems regarding the display ratio of the video (such as problems of objects becoming narrower or wider in the output image).
If the aspect ratio is required to be constant, then the horizontal and vertical scaling ratios must be the same, and remnant areas on the display device will be replaced by a specific background color. For example, if the resolution of an input video image is 640*480, and the resolution of a display device is 1280*1024, then the horizontal part and vertical part of the input frame can both be scaled up to be two times larger to become a resolution of 1280*960, and a final output frame with a resolution of 1280*1024 will be formed by adding 32 background scan lines above and below respectively. In this method, output timing of the conventional display device will be “maintaining the same display time of every horizontal scan line”, i.e. the conventional display device requires the same time to display each scan line of the 64 background scan lines as to display each scan line of the 960 output scan lines (including image data). However, in order to avoid overflow or underflow occurring in the video data, a larger buffering space is usually required to be utilized in the conventional display device, for example, more line buffers or frame buffers are required in the conventional display device. The system cost will therefore increase significantly, which is a definite disadvantage for manufacturers